Resists with enhanced sensitivity and contrast

ABSTRACT

A PMGI bilayer resist for integrated circuit fabrication having increased sensitivity to light and formed by the addition of cyclic anhydrides to the resist and the formation of an accompanying bilayer resist structure of a portable conforming mask having a desirable undercut profile for lift-off of patterned metallic circuitry.

FIELD OF INVENTION

The invention relates to the inclusion of additives inpolydimethylglutarimide (PMGI) containing resist compositions andmethods for using such compositions as single layer resists in portableconformable mask and lift-off processes. More particularly, the presentinvention is concerned with the use of certain cyclic anhydrides toenhance the sensitivity of PMGI resist compositions.

BACKGROUND ART

In the additive process of depositing patterned metal films known as thelift-off process which entails depositing a resist film on a substrateand transferring a pattern to such film followed by the blanketdeposition of a thin layer of metal in order to reproduce the patternwith the metal. The resist serves as a deposition mask separating thedesired metal pattern from the excessive metal by the vertical thicknessgap of 1-2 μm (i.e., the resist thickness). Generally the resistthickness is twice the thickness of the metal film deposition thickness.Since it is difficult to create a desirable undercut profile in a singleresist layer, multiple resist layer processes have been developed inwhich a bilayer or trilayer structure is sequentially patterned byseries of exposure and development steps using ultraviolet radiation(portable conformable mask (PCM) process as is set forth in U.S. Pat.No. 4,211,834 to Lapadula et al.) or a reactive ion etch process using apatterned metallic or other inorganic masking layer over a polymericlayer for pattern transfer and metal deposition (see, for example theprocess described in U.S. Pat. No. 3,873,361 to Franco et al.)

In a typical PCM process as shown in FIG. 2, a substrate 10 upon which aline or feature is to be placed has a planarizing underlayer 12deposited or coated thereon. The underlayer is a deep UV resistcomposition such as the polyglutarimide sold commercially as Shipley(SAL) resist. This underlayer is patternable with deep UV (200-300 nm)radiation. A top image layer 14 is deposited over the underlayer. Thetop image layer is typically a diazoquinonesensitized novolak resin(DQN) resist which is patternable in near UV (350-450 nm) radiation.

The bilayer resist is imaged with near UV radiation through a mask toform a latent image 16A in the top imaging layer as is shown in FIG. 2A.This image is developed and will have a profile 16B as may be seen inFIG. 2B.

The pattern formed by openings 16B in the top layer 14 serves as acontact printing mask to enable flood or blanket deep UV exposure totransfer a latent image 16C into underlayer 12. This image is developedas is best seen in FIG. 2C to form the profile of the opening 16D whichis characterized in having sloped foot 18. This profile serves as thedeposition and lift-off mask for forming Al-Cu metallurgy patterns ofthe order of 0.5-2.5 μm in width and 0.5-1.0 μm in thickness onsemiconductor substrates. The profile formed using a typicaldiazonaphthoquinone sensitized AZ-1350 (DQN)/PMGI bilayer structuresuffers from a protruding foot at the PMGI-silicon wafer interface.

Metallization may be by any conventional method including evaporation,sputtering, or the like to provide a metallized structure as is shown inFIG. 2D with blanket metal 20 covering the bilayer resist and the metalline or feature 20' on the substrate.

FIG. 2E shows the completion of metal lift off wherein the line orfeature 20' has a protruding fence metal which may cause shorts ordielectric breakdown between it and adjacent conductive features. Theprotruding foot of resist leads to undesirable extra metal fences 22 atthe edges of the unlifted metal circuitry. The extraneous metal tipsinduce short circuiting by contact with adjacent circuit lines.

An improved process was provided in U.S. Pat. No. 4,814,258 to Tam whichis set forth in FIG. 3. The steps and materials are the same as those inFIG. 2 except before the planarizing underlayer 12 was soaked inchlorobenzene to increase its solubility in the developer with respectto the photoresist layer 14 which is the applied and which is exposedand developed to give the image 16B as is shown in FIG. 3A.

The image 16D is undercut at the top to enable the imaging layer 14 tooverhang the planarizing layer 12 and leaves somewhat of a foot 18A.This foot leaves the opportunity for fencing as was seen in FIG. 2. Theresulting profile 16 after the chlorobenzene soak exhibits lateralundercut 18A at the DQN/PMGI interface which in the case of closelyspaced metal lines (FIG. 3B) defined by the DQN/PMGI interface tends tofurther undercut at the DQN/PMGI interface and finally topple the resiststructure. The protruding foot at the PMGI/silicon interface is noteffectively removed. Secondly, the process of soaking an organic resistlayer in a solvent involves a separate process step using chemicals of aflammable and toxic nature with the added cost of storage, safety, anddisposal. Thirdly, the soak process is isotropic and subject to manyfactors of diffusion such as the prebake conditions of the PMGI layer,the time, temperature of soaking, the shelf or use life of thechlorobenzene bath, and the purity of the chlorobenzene soakingmaterial.

The formation of an image in the PMGI planarizing layer involves a deepUV exposure of a considerable dose of the order of 500-2000 mJ/cm² andexposure times of several minutes in order to induce sufficientsolubility of the PMGI in alkaline developers. Long exposure times withlarge 200 mm diameter wafers can add extra costs when using a PCMprocess. Attempts to decrease the exposure time of positive resists ofthe DQN type have used additives of alkaline soluble substances such asacids (U.S. Pat. No. 4,009,033). The addition of these acids alsosignificantly result in loss of unexposed resist resulting in morepinholes, and insufficient step coverage over topography. Shorterexposure times at the expense of film thinning is to be avoided. Inanother system, diazoquinone sensitizers are added directly to the PMGIresist (U.S. Pat. No. 4,524,121). High doses are still required(1500-3000 mJ/cm² in the near UV region). For a PCM application, thepresence of a diazoquinone in the novolak toplayer and in the PMGIunderlayer provides no discrimination to the near UV light used to imagethe DQN toplayer. The diazoquinone in the PMGI layer would also beexposed by the near UV light resulting in sloping resist profilesunsuitable for lift-off. The toplayer of DQN would also have to be >1.5μm thick to avoid imaging the PMGI layer with near UV light. Forexposure tools of high numerical aperture, the depth of focus of thetools is close to 1.5 μm in range and thus thick resist films >1.5 μm inthe DQN imaging layer are to be avoided since blurred images will beproduced.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a faster PMGI resist forshorter exposure time, higher process throughput, and lower net costs tometal pattern definition.

It is a further object of the present invention to provide a simplifiedlower cost and a new and novel improved bilayer PCM process whichfacilitates the formation of closely spaced micron metal lines forhigher density circuitry with improved process yields.

Furthermore, it is an object of the present invention to provide a newand improved bilayer liftoff process wherein uniform lateralundercutting of the PMGI layer can be achieved without affecting theuppermost portion of the PMGI underlayer and without affecting theimaged toplayer of the DQN resist.

These and other objects and advantages are achieved by addition ofcyclic anhydride compounds to the PMGI resist which result in increasedphotospeed and desirable profile features. Other objects and advantagesof the present invention will become more apparent to those skilled inthe art upon consideration of the accompanying drawings, examples, andclaims.

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BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a liftoff pattern in accordance with the invention.

FIGS. 2A, B, C, D and E illustrate a prior art bilayer PCM resistlift-off process.

FIGS. 3A and B illustrate a solvent enhanced prior art PCM lift offprocess.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 2, the steps of the prior art of using PMGI layer12 in a PCM bilayer process is shown using a DQN resist 14 of the typedescribed in U.S. Pat. Nos. 3,046,118; 3,106,465, and 3,402,044. As isbest seen in FIG. 2B, the image 16B produced in the DQN layer 14 on asilicon substrate 10, is transferred through the PMGI layer 12 by deepUV exposure and development in tetramethyl ammonium hydroxide (TMAH)developer. The protruding foot 18 of the PMGI resist leads to excessivemetal fences in subsequent metal deposition and lift off as shown inFIGS. 2D and 2E. The metal 22 at the edges of the deposited metal 20leads to metal protrusions which can result in line short circuiting.The fence like protrusions can also break off and redeposit as freemetal pattern definition. Steeper resist profiles in the PMGI layerwithout protruding feet are desired.

The process of U.S. Pat. No. 4,814,258 as illustrated in FIG. 3 involvesa chlorobenzene soak of the PMGI layer 12 prior to the application ofthe DQN layer 14. This process leads to a topside undercut 18A of thelayer 12 as seen in FIG. 3A. This process does not eliminate the "feet"which produce unwanted metal "fences". If the process is extended toclosely spaced features (pitch of 1-2 μm lines<4 μm) as shown in FIG.3B, the lift off profile 22A in the PMGI layer 12 is subject to patterntoppling due to the lack of sufficient support at 22A to withstand thestress of the deposited metal. The support 22A is too narrow in width("necking") to support an upper structure of DQN pattern and depositedmetal.

The present invention enables the formation of stable profiles forlift-off with uniform PMGI supports 12B having a sufficient width andcross section to provide the stable structures, as shown in FIG. 1. Ithas been found that certain additives to the PMGI resist facilitate theformation of steep undercut supports 12B without the protruding feet 18as occurs in FIG. 2C, for example. The additives have also been found todecrease the exposure time of the PMGI resist without loss of resistcontrast (resolution). Prior attempts to increase the sensitivity ofPMGI have included using lower molecular weights or the addition ofdiazoquinone sensitizers. Both attempts result in poor PMGI resistancewhen the DQN layer is developed in alkaline developers and the loss ofthe PMGI contrast (resolution) and exposure process latitude (steepchange in linewidth with overdose).

The novel additives of this invention are cyclic anhydrides which may beused alone or in combination from 10-40% by weight with respect to thePMGI component of the liquid resist formulation of PMGI dissolved in acasting solvent

The preferred cyclic anhydrides of the invention include succinicanhydride, 5-norbornene-2,3-dicarboxylic anhydride and1,4,5,6,6,7-hexachloro-5-norbornene-2, 3-dicarboxylic anhydride. Theseanhydrides mix well with the underlayer resist, but do not bleed or mixinto the imaging resist. These characteristics are useful in preventingfencing.

In Table 1, the results of the evaluation of anhydride and otheradditives tested for contrast, photospeed, and suitability of lift-offprofiles are given, sloped as in FIGS. 2 and 3 or steep like in FIG. 1.For contrast and photospeed measurements, cast films of SHIPLEY SAL PMGIresist with and without 20% by weight additives were evaluated byprebaking the cast films at 215° C. for 30 minutes, exposed to a deep UVsource of a FUSION Microlite, and developed in 0.147 N TMAH. Filmthicknesses were measured of both the remaining exposed and unexposedfilm for contrast and dose to clear (Eo) for photospeed values. Forlift-off profiles, the PMGI layer with and without additives wereprocessed in a conventional manner using a DQN process sequence andevaluated by SEM for metal profiles with no sidewall metal fencing alongthe walls of the metal circuit lines.

                  TABLE 1                                                         ______________________________________                                        EFFECT OF ADDITIVES TO PMGI ON LIFT-OFF                                                REL DUV               RESIST/METAL                                   ADDITIVE EXP TIME   CONTRAST   PROFILE                                        ______________________________________                                        PMGI ctl 1.0        1.8        sloped                                         A        1.0        2.1        sloped                                         B        1.3        2.2        sloped                                         C        0.8        2.2        steep                                          D        0.6        2.5        steep                                          E        0.8        0.8        steep                                          ______________________________________                                         A = 2,2dimethoxyacetophenone                                                  B = Dichlorodimethylhydantoin                                                 C = 5Norborene-2,3-dicarboxylic anhydride                                     D = 1,4,5,6,6,7Hexachloro-5-norbornene-2,3-dicarboxylic anhydride (HONDA)     E = Succinic anhydride                                                   

For a portable conformable mask process, the proper exposure dose forthe PMGI layer is determined to be when there is no image bias betweenthe top DQN layer 14 of FIG. 2 and the PMGI layer 12 of FIG. 2. For thePMGI layer with no additive , an exposure time of 240 secs. wasdetermined. With PMGI and the HONDA additive, an exposure dose of 100secs. was noted for suitable steep profiles with no protruding feet 18in FIG. 2C. A substantial reduction in exposure time for fasterthroughput and suitable profiled for closely spaced metal features wasachieved using the addition of the invention. Without the additive butwith the chlorobenzene soaking step, the PMGI layer begins to undercutsuch as 18A in FIG. 3A at the PMGI/DQN interface. Secondly, if the PMGIlayer is overdeveloped, the DQN layer can topple due to lack ofsubstructural support FIG. B. With the HONDA and the other suitableadditives of the invention, no necking and thus lift-off structures canbe produced with overdevelopment of the PMGI layer and provides widerprocess latitude. These improvements can result in a more costeffectivePCM process for advanced semiconductor chip manufacture.

The amount of additive was found to be most effective at 20-30% byweight with no change in exposure dose. In Table 2, the results of aphotospeed and contrast study of HONDA added to PMGI are given.

                  TABLE 2                                                         ______________________________________                                        CONTRAST AND PHOTOSPEED STUDY OF HONDA                                        IN PMGI                                                                       Wt % HONDA   REL PHOTOSPEED CONTRAST                                          ______________________________________                                        none         1.0            1.8                                               10.0         0.7            2.1                                               20.0         0.5            2.4                                               30.0         0.5            2.4                                               ______________________________________                                    

It is obvious to those skilled in the art that many modifications may bemade within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all modifications withinthe scope of appended claims.

We claim:
 1. In a portable conformable mask or lift off process of thetype having a top layer and an underlayer, which process comprises thesteps ofapplying, to a substrate, an underlayer, applying, to theunderlayer, a top layer, patternwise exposing the top layer to actinicradiation to form a latent image in the top layer, developing the toplayer latent image to form a relief image in the top layer, floodexposing the underlayer to optically transfer the relief image from thetop layer and thus form a latent image in the underlayer, and developingthe underlayer latent image to form a bilayer relief image, wherein theimprovement consists of: applying, to a substrate, an underlayercomposition comprising a polydialkylglutarimide in admixture with anadditive selected from the group consisting of substituted andunsubstituted 5-norbornene-2,3-dicarboxylic anhydrides and substitutedand unsubstituted succinic anhydrides.
 2. The process of claim 1 whereinthe lateral development of the underlayer composition is approximatelyequal throughout the depth of the film.
 3. The process of claim 2wherein the relative photospeed of the underlayer composition isincreased by a factor about 2.0.
 4. The process of claim 1 wherein thetop layer consists of a diazoquinone novolak resist top layer.